Semiconductor memory device

ABSTRACT

According to an embodiment, a semiconductor memory device comprises: a semiconductor substrate; a memory cell array configured having a plurality of memory units, each of the memory units including a plurality of memory cells connected in series, the plurality of memory cells being stacked, the plurality of memory units involving a first memory unit and a second memory unit; and a plurality of bit lines connected to ends of each of the memory units in the memory cell array. The first memory unit and the second memory unit are arranged in a staggered manner by the first memory unit being displaced in a row direction with respect to the second memory unit by an amount less than an arrangement pitch in a row direction of the first memory unit or the second memory unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. application Ser. No. 14/307,196, filedJun. 17, 2014 which is a continuation of U.S. application Ser. No.13/524,750, filed Jun. 15, 2012 (now U.S. Pat. No. 8,787,061), and isbased upon and claims the benefit of priority from the prior JapanesePatent Application No. 2011-135093, filed on Jun. 17, 2011, the entirecontents of each of which are incorporated herein by reference.

BACKGROUND

1. Field

The embodiments described herein relate to a semiconductor memorydevice.

2. Description of the Related Art

In recent years, several semiconductor memory devices having memorycells disposed three-dimensionally (stacked type semiconductor memorydevices) have been proposed to increase the degree of integration ofmemory.

In one known example of such a stacked type semiconductor memory device,semiconductor pillars are formed extending in a perpendicular directionwith respect to a semiconductor substrate, and word lines disposed inmultiple layers in the perpendicular direction are connected to sidesurfaces of those semiconductor pillars via charge storage layers,thereby configuring a memory cell unit having memory cells connected inseries in the perpendicular direction. The semiconductor pillars aredisposed in a matrix in a column direction and a row direction on thesemiconductor substrate, and bit lines are disposed along thesemiconductor pillars aligned in the column direction. The chargestorage layers are formed continuously along the side surfaces of thesemiconductor pillars, hence manufacture is easy and appropriate forincreasing integration. Improvements in manufacturing technology areexpected to result in further improvements in performance due tominiaturization in this kind of stacked type semiconductor device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a semiconductormemory device according to a first embodiment.

FIG. 2 is a circuit diagram of part of a memory cell array in thesemiconductor memory device according to same embodiment.

FIG. 3 is a perspective view of part of the memory cell array in thesemiconductor memory device according to same embodiment.

FIG. 4 is a cross-sectional view of part of the memory cell array in thesemiconductor memory device according to same embodiment.

FIG. 5 is a plan view of part of the memory cell array in thesemiconductor memory device according to same embodiment.

FIG. 6 is a plan view of part of a memory cell array in a semiconductormemory device according to a comparative example.

FIG. 7 is a schematic perspective view showing part of the semiconductormemory device according to same embodiment.

FIG. 8 is a perspective view of part of a memory cell array in asemiconductor memory device according to a second embodiment.

FIG. 9 is a plan view of part of the memory cell array in thesemiconductor memory device according to same embodiment.

FIG. 10 is a circuit diagram of part of the memory cell array in thesemiconductor memory device according to same embodiment.

FIG. 11 is a plan view of part of a memory cell array in a semiconductormemory device according to a third embodiment.

FIG. 12 is a perspective view of part of a memory cell array in asemiconductor memory device according to a fourth embodiment.

FIG. 13 is a block diagram showing a schematic configuration of asemiconductor memory device according to a fifth embodiment.

FIG. 14 is a block diagram showing a schematic configuration of asemiconductor memory device according to a sixth embodiment.

FIG. 15 is a block diagram showing a schematic configuration of asemiconductor memory device according to a seventh embodiment.

DETAILED DESCRIPTION

A semiconductor memory device according to an embodiment comprises: asemiconductor substrate; a memory cell array configured having aplurality of memory units; and a plurality of bit lines connected toends of each of the memory units in the memory cell array. Each of thememory units include a plurality of memory cells connected in series.The plurality of memory cells are stacked. The plurality of memory unitsinvolve a first memory unit and the second memory unit. The plurality ofbit lines involving a first bit line and a second bit line which isadjacent to the first bit line. The first bit line is connected to thefirst memory unit, and the second bit line is connected to the secondmemory unit. The first memory unit and the second memory unit arearranged in a staggered manner by the first memory unit being displacedin a row direction with respect to the second memory unit by an amountless than an arrangement pitch in a row direction of the first memoryunit or the second memory unit.

Embodiments of the semiconductor memory device are described below withreference to the drawings.

First Embodiment Overall Configuration

First, an overall configuration of a semiconductor memory deviceaccording to a first embodiment is described with reference to FIG. 1.FIG. 1 is a block diagram showing the configuration of the semiconductormemory device according to the first embodiment.

As shown in FIG. 1, the semiconductor memory device according to thefirst embodiment includes a memory cell array 11 that comprises aplurality of memory blocks MB. The plurality of memory blocks MB#0-#jare aligned in a column direction (direction in which bit lines BLextend). The memory blocks MB#0-#j are connected to the bit linesBL1-BLn and a source line SL. All the memory blocks MB#0-#j share thebit lines BL1-BLn and the source line SL. The plurality of memory blocksMB#0-#j are connected to a plurality of word lines WL and a plurality ofselect gate lines SGD and SGS extending in a row direction. The wordlines WL are connected to a row decoder 12, and the select gate linesSGD and SGS are connected to a row decoder 13. Moreover, the bit linesBL are connected to a column decoder 15 via a sense amplifier 14.

The row decoder 12 selects the word lines WL based on a row addressoutputted from an address pre-decoder 16. The row decoder 12 transfers avoltage generated by a word line driver 17 to, respectively, a selectedword line WL and an unselected word line WL.

The row decoder 13 selects a source side select gate line SGS and adrain side select gate line SGD corresponding to a memory unit MU shownin FIG. 2 activated based on the row address outputted from the addresspre-decoder 16. The row decoder 13 transfers a gate voltage generated bya select gate line driver 18 to a selected source side select gate lineSGS and drain side select gate line SGD.

The column decoder 15 decodes a column address signal outputted from theaddress pre-decoder 16 and performs input/output control of data. Thesense amplifier 14 senses and latches data of the bit line BL selectedby the column decoder 15. A controller 19 receives a signal forexecuting a read/write/erase operation and so on, from an addresscommand register not illustrated, and controls an internal voltagegenerating circuit not illustrated that generates various voltagesrequired in core operation, according to a certain sequence. Note that aperipheral circuit of the row decoders 12 and 13, the sense amplifier14, the column decoder 15, and so on, may be formed directly below thememory cell array 11.

[Memory Cell Array]

Next, a configuration of the memory cell array 11 is described withreference to FIGS. 2-5. FIG. 2 is a circuit diagram showing part of onememory block MB in the memory cell array 11.

As shown in FIG. 2, the memory block MB includes a plurality of memoryunits MU. These memory units MU are arranged in a matrix and arrangedstaggered in the column direction.

One ends of the memory units MU are connected alternately to two bitlines BLj and BLj+1 disposed along these memory units MU. Specifically,an example is described of the memory units MU (MU1-MU4) connected tothe bit lines BL1 and BL2. The memory units MU1-MU4 are arrangedstaggered in the column direction.

One end of the memory unit MU1 is connected to the bit line BL1. One endof the memory unit MU2 which is misaligned in the row direction withrespect to the memory unit MU1 is connected to the bit line BL2.Similarly, one end of the memory unit MU3 having the same row directionas the memory unit MU1 is connected to the bit line BL1. One end of thememory unit MU4 which is misaligned in the row direction with respect tothe memory unit MU3 is connected to the bit line BL2.

Other ends of each of the memory units MU are commonly connected to thesource line SL.

Each of the memory units MU includes a memory string MS configured froma plurality of memory transistors MTr1-MTr8 connected in series and aback gate transistor BTr connected between the memory transistors MTr4and MTr5, and, at the two ends of the memory string MS, a source sideselect transistor SSTr and a drain side select transistor SDTr. Thememory transistors MTr1-MTr8 each have a MONOS structure, for example,that includes: a charge storage layer (for example, an insulating film)formed on a side surface of a semiconductor body via a gate insulatingfilm; an insulating film (insulating film having a higher permittivitythan the charge storage layer) formed on a side surface of the chargestorage layer; and a control gate formed on a side surface of thisinsulating film. The memory transistors MTr1-MTr8 each change theirthreshold voltage by storing a charge in their charge storage layer. Asa result, the memory transistors MTr1-MTr8 each store informationcorresponding to this threshold voltage.

In the memory block MB, gates of memory transistors MTrj aligned in therow direction are commonly connected to a word line WLj extending in therow direction. In addition, in one memory block MB, word lines WLjconnected to corresponding memory transistors MTrj in each of the memoryunits MU are commonly connected. Moreover, gates of back gatetransistors BTr in the memory units MU are commonly connected to theback gate line BG.

In the memory block MB, gates of each of the drain side selecttransistors SDTr aligned in the row direction are commonly connected tothe drain side select gate line SGD extending in the row direction.Moreover, in the memory block MB, gates of each of the source sideselect transistors SSTr aligned in the row direction are commonlyconnected to the source side select gate line SGS extending in the rowdirection.

Next, a structure of the memory cell array 11 according to the firstembodiment is described with reference to FIGS. 3-5.

FIG. 3 is a perspective view showing a configuration of part of thememory cell array 11.

As shown in FIG. 3, the memory cell array 11 is configured comprising aU-shaped pillar semiconductor layer 30 having both ends (source end anddrain end) extending upwardly in a perpendicular direction on asemiconductor substrate 20, with the back gate transistor BTr as afolded part. The semiconductor layer 30 is disposed having its both ends(upper ends) aligned in the column direction, and is disposed inplurality in a matrix in the column direction and the row direction. Asshown in an enlarged view of part of a cross-section of thesemiconductor layer 30 in FIG. 4, the semiconductor layer 30 isconfigured by a pillar semiconductor body 31, a tunnel insulating layer32 covering a side surface of the semiconductor body 31, a chargestorage layer 33, and a block insulating layer 34. Employable as thetunnel insulating layer 32 and the block insulating layer 34 is, forexample, silicon oxide (SiO₂) or the like. Employable as the chargestorage layer 33 is, for example, silicon nitride (SiN) or the like.

A back gate BG is disposed on the semiconductor substrate 20. The backgate transistor BTr is formed by this back gate BG and the folded partof the semiconductor layer 30. The folded part herein is described usingFIG. 3. The semiconductor layer 30 includes a first pillar portion 30A,a second pillar portion 30B, and a folded portion 30C. The folded partrefers to this 30C in FIG. 3.

Stacked around the first pillar portion 30A, via insulating layers, inorder from the semiconductor substrate 20 side in an upwardlyperpendicular direction, are conductive layers forming the word linesWL4, WL3, WL2, WL1, and the source side select gate line SGS. Theseconductive layers are connected to a side surface of the semiconductorlayer 30. Stacked around the second pillar portion 30B, via insulatinglayers, in order from the semiconductor substrate 20 side in an upwardlyperpendicular direction, are conductive layers forming the word linesWL5, WL6, WL7, WL8, and the drain side select gate line SGD. Theseconductive layers are connected to a side surface of the semiconductorlayer 30. As a result, the memory transistors MTr1-8 are formed havingthe word lines WL1-8 as control gates, and the U-shaped semiconductorbody 31 as a channel body. In addition, the source side select gate lineSGS, the drain side select gate line SGD, and the back gate BG have theU-shaped semiconductor layer 30 as a body to configure, respectively,the source side select gate transistor SSTr, the drain side select gatetransistor SDTr, and the back gate transistor BTr.

That is, the memory transistors MTr1-8 and the back gate transistor BTrconfigure the memory string MS, having a stacking direction as a longdirection. Moreover, the memory string MS, the drain side select gatetransistor SDTr, and the source side select gate transistor SSTrconfigure the memory unit MU. A source side of the memory unit MU, thatis, one of the ends of the semiconductor layer 30, is connected to thesource line SL. A drain side of the memory unit MU, that is, the otherof the ends of the semiconductor layer 30, is connected to the bit lineBL via a bit line contact BC. The bit line BL and the bit line contactBC are formed thinner than the semiconductor layer 30.

FIG. 5 is a plan view showing a configuration of part of the memory cellarray.

As shown in FIG. 5, in the semiconductor memory device according to thepresent embodiment, the memory units MU are disposed in a staggeredmanner. That is, the memory units MU adjacent in the row direction aredisposed with a spacing of a pitch P1, and the memory units MU adjacentin the column direction are disposed at positions shifted with respectto each other in the row direction by an increment of a half pitch P2(half of the pitch P1). In addition, the bit lines BL are arranged inthe row direction with the pitch P2. Therefore, the memory units MUadjacent in the column direction are respectively connected to differentbit lines BL.

As a comparative example, FIG. 6 shows a plan view of a generalsemiconductor memory device. In the comparative example, a pitch in therow direction of the bit lines BL and the memory units MU is configuredequal.

The memory unit MU is formed in a memory hole. The memory hole is formeddeeply in the stacking direction in the word lines WL and insulatinglayers. Moreover, the memory unit MU has the semiconductor layer formedhaving a charge storage layer and insulating layer formed on itssurface. Hence, miniaturization of the memory unit MU is not as easy asfor the bit line BL. In contrast, the bit line BL can be lithographed bysimple line-and-spacing, hence further miniaturization can be realizedeasily by, for example, a sidewall transfer process. Specifically, thebit line BL can be formed with a width of about 1/n times the width ofthe semiconductor layer 30 (where n is a natural number).

Focusing on this point, as shown in FIG. 5, the semiconductor memorydevice according to the present embodiment, by having the memory unitsMU disposed in a staggered manner and having the pitch P2 in the rowdirection of the bit lines BL set to ½ of the pitch P1 of the memoryunits MU, enables simultaneous access to two times the number of bitlines BL as in the comparative example.

As described above, the present embodiment has twice the number of bitlines BL connected to the same number of memory units MU as in thecomparative example. Therefore, the present embodiment, bysimultaneously selecting the select transistors SDTr and SSTr in thememory units MU respectively connected to different adjacent bit lines,for example, connected to BL1 and BL2 by the row decoder 13 shown inFIG. 1, results in data number capable of being read or written in asingle read operation (page length) being two times that of aconventional example and thereby enables read speed to be dramaticallyimproved. As a result, although conventionally it was required toarrange additional memory units MU in the word line WL direction toincrease page length, the present embodiment allows page length to beincreased without increasing circuit area.

Note that since the bit lines BL are connected to the sense amplifier14, there is a risk that increasing the number of bit lines leads to anincrease in circuit area. However, the semiconductor memory deviceaccording to the present embodiment is a semiconductor memory device ofthe so-called Pipe type employing the U-shaped semiconductor layer 30 asa channel body of the memory unit MU. Hence, it is possible to bring thewiring together above the memory cell array 11 and form the senseamplifier 14 under the memory cell array 11 as shown in FIG. 7.Therefore, circuit area is determined by area of the memory cell array11, and it can be prevented that circuit area increases to be largerthan area of the memory cell array 11.

Second Embodiment

Next, a configuration of a semiconductor memory device according to asecond embodiment is described with reference to FIGS. 8-10. FIG. 8 is aperspective view showing a configuration of part of a memory cell array11 according to the second embodiment; FIG. 9 is a plan view of samememory cell array 11; and FIG. 10 is a circuit diagram of same memorycell array 11. Note that identical symbols are assigned toconfigurations identical to those in the first embodiment, anddescriptions thereof are omitted.

The present embodiment differs from the first embodiment in having thesource side select gate line SGS and the drain side select gate line SGDshared between the memory units MU adjacent in the column direction.That is, focusing on a certain memory unit MU shown in FIG. 9, thememory units MU disposed to one side of this certain memory unit MU inthe column direction share with the certain memory unit MU, for example,the source side select gate line SGS, and the memory units MU disposedto the other side of this certain memory unit MU in the column directionshare with the certain memory unit MU, for example, the drain sideselect gate line SGD.

Accordingly, as shown in FIG. 10 for example, in the second embodiment,the drain side select gate lines SGD1 and SGD2 in the first embodimentshown in FIG. 2 are commonly connected to become a drain side selectgate line SGD1′. Similarly, in the second embodiment, the source sideselect gate lines SGS2 and SGS3 in the first embodiment shown in FIG. 2are commonly connected to become a source side select gate line SGS2′.Activating the drain side select gate line SGD1′ and the source sideselect gate lines SGS1′ and SGS2′ results in n bits of data beingsimultaneously accessed from the upper two columns of memory units MU inFIG. 10 via the bit lines BL1-BLn.

As shown in FIG. 9, the second embodiment allows a spacing of the memoryunits MU adjacent in the column direction to be narrowed as shown by P3in FIG. 9, more than in the first embodiment where the select gates SGSand SGD of the memory units adjacent in the column direction are eachprovided independently. That is, in the present embodiment, the spacingbetween the memory units MU in the column direction is smaller than aspacing between the memory units in the row direction. As a result, inthe second embodiment, the word lines WL can be miniaturized comparedwith the conventional example. In other words, a length in the columndirection of the plurality of word lines WL is smaller than a sum of twoclosest distances in the column direction from sides of thesemiconductors 30 to both ends of the word lines WL, two diameter'sworth of the semiconductor 30, and a spacing between the semiconductors30 adjacent in the column direction. In a structure where a plurality ofplate-shaped word lines WL are stacked as in the present embodiment,parasitic capacitance is generated between the word lines WL overlappingin the stacking direction. However, in the present embodiment, narrowingthe spacing between the memory units MU allows this parasiticcapacitance to be reduced, thereby leading to increased powerconsumption saving and speeding up of operations. In addition, the factthat the word lines WL are capable of being miniaturized allows adimension in the column direction of the memory cell array 11 to beshortened, and a length in the column direction of the bit lines BL alsoto be shortened. This too enables increased power consumption saving andspeeding up of operations to be achieved.

Third Embodiment

FIG. 11 is a plan view showing part of a semiconductor memory deviceaccording to a third embodiment.

In the first and second embodiments, the memory units MU adjacent in thecolumn direction were disposed misaligned by an increment of a halfpitch in the row direction. However, in the third embodiment, the memoryunits MU adjacent in the column direction are disposed misaligned by anincrement of a finer pitch than this, for example, a pitch P4 which is ⅓of the pitch P1 in the row direction of the memory units MU. Such aconfiguration results in page length being further increased and enablesfurther improvement in read speed.

Fourth Embodiment

Next, a semiconductor memory device according to a fourth embodiment ofthe present invention is described. FIG. 12 is a perspective viewshowing a configuration of part of the semiconductor memory deviceaccording to the present embodiment. In the first through thirdembodiments, a U-shaped type semiconductor layer 30 was employed as achannel body of the memory units MU. However, in the present embodiment,a pillar type (I type) semiconductor layer 40 is employed as a channelbody of the memory units MU. In such a configuration, a back gatetransistor BTr is not provided, and the source line SL is disposed at alower part of the memory string MS.

Such a configuration also has the memory units MU adjacent in the columndirection displaced by ½ of the pitch in the row direction, therebymaking it possible to obtain similar advantages to those of the firstembodiment. Note that similarly to the third embodiment, the presentembodiment also allows the memory units MU adjacent in the columndirection to be configured misaligned by an increment of a finer pitchthan a half pitch of the pitch in the row direction.

Fifth Embodiment

Next, a fifth embodiment is described with reference to FIG. 13. FIG. 13is a block diagram showing a schematic configuration of a semiconductormemory device according to the present embodiment.

In the present embodiment, similarly to in the first embodiment,disposing the memory units MU staggered results in approximately twotimes as many bit lines BL being allocated as the number of memory unitsMU arranged in the row direction. In the first embodiment, the samenumber of sense amplifiers as the number of memory units MU in the rowdirection must be provided, for example. That is, the same number ofsense amplifiers as bit lines BL connected to the memory units MU becomenecessary. However, in the present embodiment, since one sense amplifier14 is used alternately by two bit lines BL, a selecting circuit SEL isprovided between the bit lines BL and the sense amplifier 14.

In the case of this embodiment, area of the sense amplifier can besuppressed to an area similar to that in a conventional device. A senseamplifier 14 requires a greater circuit area than a select circuit. As aresult, reducing a number of sense amplifiers as in the presentembodiment allows increase in circuit area overall to be prevented. Inorder to read and write data alternately in adjacent bit lines, each ofthe memory units MU may be independently supplied with, respectively,the select gate lines SGS and SGD, or the select gate lines SGS and SGDmay be shared by a pair of the memory units MU.

Sixth Embodiment

FIG. 14 is a block diagram showing a schematic configuration of asemiconductor memory device according to a sixth embodiment. A basicconfiguration of the present embodiment is similar to that of the firstembodiment, but differs in that a differential type sense amplifier isused as the sense amplifier 14. In the present embodiment, a pair cellis configured by a pair of corresponding memory transistors MTr inmemory units MU adjacent in the column direction, and this pair cellstores data that differs logically one from another. In this case, apair of data is read from adjacent bit lines BL and differentialdetection is performed by the sense amplifier 14.

This embodiment allows configuration of a memory resistant to noise,disturbance, and the like.

Seventh Embodiment

FIG. 15 is a block diagram showing a schematic configuration of asemiconductor memory device according to a seventh embodiment. In thepresent embodiment, one of two bit lines BL is connected to the senseamplifier 14, and the other of the two bit lines BL is grounded and usedas a shield line.

The present embodiment, while setting the number of bits of a page thatare read at one time to be the same as in a conventional device, allowsevery other bit line BL to be used as a shield, hence enables evengreater stability of data read to be achieved.

OTHER

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

1. (canceled)
 2. A semiconductor memory device, comprising: first,second and third bit lines extending in a first direction, the secondbit line being adjacent to the first bit line and the third bit line; afirst memory unit including a first select transistor, the first memoryunit being electrically connected to the first bit line; a second memoryunit including a second select transistor, the second memory unit beingadjacent to the first memory unit in a second direction crossing thefirst direction, the second memory unit being electrically connected tothe second bit line; a third memory unit including a third selecttransistor, the third memory unit being adjacent to the first memoryunit in a third direction crossing the first direction and the seconddirection, the third memory unit being electrically connected to thethird bit line; and a first conductive layer connected to a gate of thefirst select transistor, a gate of the second select transistor and agate of the third select transistor.
 3. The semiconductor memory deviceaccording to claim 2, wherein the first conductive layer extends in thethird direction.
 4. The semiconductor memory device according to claim2, further comprising a fourth memory unit including a fourth selecttransistor, the fourth memory unit being electrically connected to thefirst bit line; a fifth memory unit including a fifth select transistor,the fifth memory unit being adjacent to the fourth memory unit in thesecond direction, the fifth memory unit being electrically connected tothe second bit line; a sixth memory unit including a sixth selecttransistor, the sixth memory unit being adjacent to the fifth memoryunit in the third direction, the sixth memory unit being electricallyconnected to the third bit line; and a second conductive layer connectedto a gate of the fourth select transistor, a gate of the fifth selecttransistor and a gate of the sixth select transistor.
 5. Thesemiconductor memory device according to claim 4, wherein the first andsecond conductive layers extend in the third direction.
 6. Thesemiconductor memory device according to claim 2, wherein the firstdirection is substantially perpendicular to the third direction.
 7. Thesemiconductor memory device according to claim 2, further comprising: aselecting circuit electrically connected to the first and second bitlines.
 8. The semiconductor memory device according to claim 2, whereina first pitch is larger than a second pitch, the first pitch being adistance between the first memory unit and the second memory unit, thesecond pitch being a distance between the first bit line and the secondbit line.
 9. The semiconductor memory device according to claim 2,wherein the first memory unit comprises a U-shaped semiconductor bodyincluding two semiconductor pillars and a first portion, one end of thefirst portion being connected to one of the semiconductor pillars, theother end of the first portion being connected to another one of thesemiconductor pillars.
 10. The semiconductor memory device according toclaim 2, further comprising: a substrate; and a sense amplifier circuitprovided between the first memory unit and the substrate.
 11. Thesemiconductor memory device according to claim 2, wherein the firstmemory unit further includes a first memory transistor; the secondmemory unit further includes a second memory transistor; the thirdmemory unit further includes a third memory transistor; and a gate ofthe first memory transistor, a gate of the second memory transistor anda gate of the third memory transistor are connected to a thirdconductive layer, the third conductive layer extending in the thirddirection.